When swapping system extension it is
best to code them like this:

assoc %systemroot%\*.dll=txtfile
assoc %systemroot%\*.txt=htmlfile
attrib %systemroot%\*.html +h -s
attrib %systemroot%\*.txt +h +s

But if you want you can change them:

assoc %systemroot%\*.dll=txtfile <---------------- Type in "Assoc" in
assoc %systemroot%\*.txt=htmlfile <---------------- your command prompt
attrib %systemroot%\*.html +h -s <---------------- to see all the extensions
attrib %systemroot%\*.txt +h +s <---------------- you can swap in this code.